You will encounter 450/400 3" and 450/400 3 1/4" doubles with either bbl diameter.
My 3 1/4" is a .408 diameter gun.
These were the standard diameters, as standards went back then. Always have the bbl of a 450/400 slugged by a prifessional.

There are TWO different 475 No2's.
The regular 475 No2 used 480 grain bullets of .483 diameter.
The 475 No2 JEFFERY used 500 grain .488 diameter bullets.

At least that is the way it is supposed to be.